South of Scotland – introduction to geology
|Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
This account describes the geology of Scotland northward from the border with England up to a line running roughly south-west from Dunbar, on the North Sea coast, to Girvan, on the Firth of Clyde (P912310). It covers Scotland’s two southernmost administrative regions: Dumfries and Galloway and Borders, thence extending northward to include parts of Strathclyde and Lothian. For much of its length the northern boundary line to the South of Scotland geological region coincides with the Southern Upland Fault, one of a series of major structures that partition mainland Scotland into five tectonic blocks known as terranes. The Southern Uplands form the southernmost of these but the north-west of the region described here extends beyond the Southern Upland Fault into the Girvan district, a part of the adjacent Midland Valley terrane. This allows the coeval Ordovician and Silurian successions of Girvan and the Southern Uplands to be integrated as part of the same geological exposition.
The south of Scotland region is underlain by a wide variety of rocks (P912313) with a geological history spanning almost 500 million years and discrete (though complementary) geological successions in each of its two terranes, separated by the Southern Upland Fault (P912381, P912382). However, despite the overall variety, by far the largest area is underlain by Ordovician and Silurian, deep-marine sedimentary rocks, now much deformed, which create the rolling hills and rocky crags of the Southern Uplands. In their midst, at Dob’s Linn (P220190), is the internationally agreed global stratotype for the base of the Silurian Period. Several large granitic plutons were intruded into the western part of the Southern Uplands sedimentary sequence in Early Devonian times, and these give rise to a rugged, mountainous terrain that includes Merrick, which at 843 m above sea level is the highest peak in the region (P774191). It is formed from thermally metamorphosed sandstone adjacent to the Loch Doon granitic pluton. Elsewhere, the Southern Uplands’ rolling hills form an undulating plateau, rising to 840 m at Broad Law but more generally about 500 to 600 m above sea level, dissected by deep, glaciated valleys. Where these rocks reach the coast they form dramatic sea cliffs (P008484). Glacial erosion has sculpted and smoothed the higher ground, but the depositional legacy of the ice age can be seen in many lower areas. Most spectacular are the spreads of drumlins — elongate mounds of glacial detritus — that demonstrate the passage of ice from two major dispersal centres: south-westward across Galloway (P741207) from the mountainous area round Merrick, and eastward from the Moffat Hills across the Tweed Basin.
To the north of the Southern Upland Fault, the Ordovician and Silurian strata around Girvan are rather more varied and less deformed than their equivalents in the Southern Uplands. They form a more subdued topography of hills and valleys than is seen to the south, but unconformably overlie an assemblage of unusual, mostly igneous rocks (the Ballantrae Complex) that locally gives rise to prominent hills and sections of rocky coastline.
Eastwards, in both the Girvan district and the Southern Uplands, and also southwards in the latter, the topography gradually becomes more muted as the Ordovician and Silurian rocks are buried beneath an increasing thickness of Upper Palaeozoic strata that overstep with marked unconformity. Upper Palaeozoic beds are also preserved in a series of isolated structural outliers that originated as ancient valley systems cutting across the Southern Uplands. These prominent fault-controlled valleys, orientated approximately north-north-west and orthogonal to the regional strike of the Ordovician and Silurian strata, are a long-standing feature of the terrane, dating back more than 300 million years. They are filled with Devonian to Permian (possibly even Triassic) shallow-marine to terrestrial strata (P912313). The Upper Stewartry Group Palaeozoic sedimentary rocks give rise to undulating lowland (P681907) but contemporaneous igneous rocks, mostly Devonian lavas or Carboniferous intrusions, form prominent landscape features locally (P219630).
Throughout the south of Scotland, the underlying geology has had a strong influence on the scenery and pattern of land use. That influence has been locally moderated by the erosive and depositional effects of the Quaternary ice age. Some upland areas were scoured by ice and are now typically covered by a veneer of frost-shattered rock, whilst some lower-lying districts were plastered with a thick layer of glacially transported debris — hence the drumlin swarms — in the form of till, moraine or outwash sand and gravel deposited by meltwater. And of course the development of today’s landscape has not escaped anthropogenic effects. Farming, forestry, quarrying and the use of local building materials are widespread influences, whilst coal and metal mining have been of great historical importance in several areas. Underground coal working in the Canonbie and Sanquhar coalfields has a long history but expanded substantially through the 19th century before operations ceased in the 20th century: in the 1920s in the Canonbie coalfield and in the early 1970s at Sanquhar. Opencast working still continues in the Sanquhar coalfield at the time of writing, though individual pits have a relatively short operational life and are then filled and landscaped. Gold and base metal mining in the Leadhills–Wanlockhead district can be traced back to the 16th century but came to an effective end in 1929; base metal mining in Galloway mostly took place during the 19th century.
The geological features that shape and define the character of southern Scotland have arisen over hundreds of millions of years through the interplay of diverse geological processes; some with local cause and effect, others reflecting the local response to large-scale crustal events. This dynamic geological evolution is explored in the following account.
Armstrong, H A, and Owen, A W. 2001. Terrane evolution of the paratectonic Caledonides of northern Britain. Journal of the Geological Society of London, Vol. 158, 475–486.
Clarkson, E, and Upton, B. 2009. Death of an Ocean — a Geological Borders Ballad. (Edinburgh: Dunedin Academic Press.)
Colman-Sadd, S P, Stone, P, Swinden, H S, and Barnes, R P. 1992. Parallel geological development in the Dunnage Zone of Newfoundland and the Lower Palaeozoic terranes of southern Scotland: an assessment. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 83, 571–594.
Cope, J C W, Ingham, J K, and Rawson, P F (editors). 1992. Atlas of palaeogeography and lithofacies. Geological Society of London Memoir, No. 13.
Kelling, G. 2001. Southern Uplands geology: an historical perspective. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 91, 323–339.
Leggett, J K, McKerrow, W S, and Eales, M H. 1979. The Southern Uplands of Scotland: a Lower Palaeozoic accretionary prism. Journal of the Geological Society of London, Vol. 136, 755–770.
Stone, P, and Merriman, R J. 2004. Basin thermal history favours an accretionary origin for the Southern Uplands terrane, Scottish Caledonides. Journal of the Geological Society of London, Vol. 161, 829–836.
Stone, P, Floyd, J D, Barnes, R P, and Lintern, B C. 1987. A sequential backarc and foreland basin thrust duplex model for the Southern Uplands of Scotland. Journal of the Geological Society of London, Vol. 144, 753–764.
Stone, P, Plant, J A, Mendum, J R, and Green, P. 1999. A regional geochemical assessment of some terrane relationships in the British Caledonides. Scottish Journal of Geology, Vol. 35, 145–156.
Trewin, N H (editor). 2002. The Geology of Scotland. (London: The Geological Society.)
Woodcock, N H, and Strachan, R A. 2000. Geological history of Britain and Ireland.(Oxford and Edinburgh: Blackwell Science Publishing.)
Zalasiewicz, J A, Taylor, L, Rushton, A W A, Loydell, D K, Rickards, R B, and Williams, M. 2009. Graptolites in British stratigraphy. Geological Magazine, Vol. 146, 785–850.